1. Field of the Invention
This invention relates to switching circuits.
The invention is particularly, though not exclusively, applicable to switching circuits for electric motors and generators.
2. Description of Related Art
Electric motors and generators commonly rely on switching circuits for their control. In the case of switched reluctance motors a power converter typically provides pulses of unidirectional current in sequence to each of the phase windings. Similar sequential switching regimes are used to draw power from a switched reluctance generator.
The source of the power for the converter is typically a capacitively smoothed direct voltage source--often referred to as a `dc link`--which can be derived from a dc power supply or battery, or derived from a rectifier circuit which draws power from an ac source. The dc link is periodically switched to the windings of the switched reluctance motor/generator using semiconductor power switching devices.
Where the voltage of the dc link is relatively low, the semiconductor power switching devices through which the windings are energized have adequate voltage ratings to allow a single semiconductor device to be used in association with each phase as shown in FIG. 1. Typical power converter circuits will be known to the skilled person and a variety of power switching devices is available, including bipolar transistors and field-effect transistors, insulated gate bipolar transistors, gate turn-off thyristors, MOS-controlled thyristors, static induction thyristors and other devices having the ability to switch between a high impedance OFF state and a lower impedance ON state and vice versa.
There also exist power modules incorporating more than one semiconductor power switching device. These are also applicable to switched reluctance power converters.
In circumstances in which the dc link voltage is too large for the voltage rating of an appropriate semiconductor power switching device, a common solution is to connect two switching devices in series. This series connection of two power switching devices ideally allows each device to be exposed, both transiently and in the OFF state, to approximately half the voltage across the two devices together. However, as a practical matter series connected semiconductor switching devices are unlikely to turn on or off at precisely the same instant. Any delay in switching times of one device with respect to the other will give rise to an excessive voltage appearing across the slower or later device at switch on and across the faster or earlier device at switch off.
For example, FIG. 1 illustrates a power converter system for one phase winding 1 of a switched reluctance motor. When the unexcited winding 1 is suddenly connected to the dc link by the two power switching devices 3 and 4 which receive actuating signals at their respective control electrodes 5 and 6, the voltage across the winding changes suddenly from zero to V.sub.L, where V.sub.L is the link voltage.
If, while current is still flowing in the winding 1, one of the two power switching devices 3 and 4 is switched off (say 3) by applying or removing the appropriate control signal to its control electrode, the winding current is compelled to flow through one of a pair of diodes 7 and 8 (say 8), respectively connected between each end of the winding and the opposite voltage dc link terminal, and the other power switching device 4 which remains actuated.
The winding voltage changes suddenly from V.sub.L to approximately zero and the current in the winding varies at a rate depending primarily on the current in the winding inductance, the value of the winding inductance itself, the rate of change of the winding inductance and on the resistance of the winding.
Alternatively if, while the winding current is still flowing, both power switching devices are switched off simultaneously the winding current is compelled to flow through both diodes which are so connected to the dc link that the winding voltage changes suddenly from V.sub.L to -V.sub.L.